Bit of backstory as I don't want to drip feed.
DH and I have been NC with MIL and SIL 1 for roughly 2 years. MIL due to her refusal to accept that DH primary family are me and the DC and that coupled with her controlling behaviour. It had been going on for years slowly grounding him down and then one day She said some nasty things to him about not being able to love DD the same as DS as she looks like me and he walked away (long story short it wasn't that simple) she's never apologised and at the time couldn't understand why DH was hurt as her feelings are valid and she has every right to express them.
SIL 1 and I had a falling out 4 years ago when I cancelled babysitting for her as I was in the process of miscarrying. Apparently I was selfish as her DS would have been in bed and her sofa is more comfortable then mine anyway 
I've also got SIL 2 who I used to be close to but we don't see that regularly now due to the strained relations between DH and family.
SIL 2 came over upset yesterday as her DD has just started nursery and needed a costume for the nativity which she couldn't find. I've told her it's not a big deal and a top with the character would be fine etc but she is young and financially struggling and was upset so I offered to make one.
Not a problem I'm fairly crafty and as it turned out had all the materials. I dropped it to her last night took about 6 or so hours in between dinner etc.
SIL 1 has just text me out of the blue to say that she has seen the costume it is not fair I am playing favourites between my nieces and nephews, her DS needs to be an elf for his school play, costume due in on Monday so she will be over Saturday afternoon to pick it up incase there are any adjustments that need to be made.
WIBU to text her back to fuck off?
Nephew doesn't even know I exist as they took down all the photos etc of us from MIL house.
Or am I letting my view of her cloud my judgement and should I make it seeing as show I've made one for my niece?
I will think IANBU to refuse but I'm doubting myself now